This invention relates to a power roller for a toroidal continuously variable transmission.
In recent years, research has been carried out on types of vehicle transmission which continuously vary a speed ratio. In addition to belt type continuously variable transmissions, traction drive continuously variable transmissions which transmit power by the shear of oil film (referred to hereafter as toroidal CVT) are well known. Toroidal CVT allow an engine to be driven at optimum efficiency, and this technique is therefore of much interest as it permits a large improvement of fuel cost performance and power transmission. Jikkai Sho 62-199557 published by the Japanese Patent Office in 1987 discloses such a toroidal CVT.
According to Jikkai Sho 62-199557, a prior art toroidal CVT comprises a power roller having an inner race which transmits power between an input disk and an output disk via an oil film, an outer race supported by a trunnion (power roller supporting member), and a thrust ball bearing which supports the inner race on the outer race so that is free to rotate.
This thrust ball bearing comprises plural balls, a holder which holds these balls, and an inner race groove and outer race groove formed respectively in opposite surfaces of the inner race and outer race. Both of these race grooves have an arc-shaped cross-section, and the whole forms a ring. In the prior art, a radius of curvature ri of the inner race groove and a radius of curvature ro of the outer race groove are identical, and the pitch diameter xcfx86di of the inner race groove and the pitch diameter xcfx86do of the outer race groove are also identical.
However, in the prior art toroidal continuously variable transmission, the direction of the rotation axis of the balls of the thrust ball bearing cannot be controlled. Therefore, if the vehicle is driven for a long-time while the spin angular velocity of the balls relative to the inner race groove is large, the rolling contact parts of the balls and inner race groove generate heat, and the temperature of the inner race groove rises. The inner race transmits power from the input disk to the output disk by traction drive. A traction coefficient degreases together with temperature rise of an oil film between the inner race and input disk or between the inner race and output disk, and when the temperature of the inner race rises, power transmission efficiency declines.
It is therefore an object of this invention to provide a toroidal continuously variable transmission which can prevent decrease of power transmission efficiency due to temperature rise of an oil film of a power transmission part.
In order to achieve above objects, this invention provides a power roller for a toroidal continuously variable transmission, comprising an inner race for transmitting power between an input disk and an output disk via an oil film, an outer race supported by a power roller support member, and a thrust ball bearing for supporting the inner race on the outer race such that it is free to rotate.
The thrust ball bearing has an inner race groove having an arc-shaped cross-section formed in the inner race, and an outer race groove having an arc-shaped cross-section formed in the outer race, an annular space defined by the inner race groove and outer race groove, plural balls disposed in this annular space, and a holder supporting the balls.
In the contact part between each of the balls and inner race groove, a first spin, which is the slip between each of the balls and inner race groove wherein each of the balls and inner race groove rotate relative to each other, occurs, in the contact part between each of the balls and outer race groove, a second spin, which is the slip between each of the balls and outer race groove wherein each of the balls and outer race groove rotate relative to each other, occurs, and the direction of the rotation axis of the balls defined by the balance between the frictional moment of the first spin and the frictional moment of the second spin, is set to a direction in which the angular velocity of the first spin is less than the angular velocity of the second spin.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.